1. Field of the Invention
This invention relates to a method and apparatus for inspecting fastener holes and, more particularly, to a system for determining the distance between a capacitive probe and the surface of a fastener hole to evaluate compliance of the fastener hole with specified dimensional standards.
2. Discussion of the Prior Art
Modern aircraft can have as many as a million or more structural fasteners including straight and tapered fasteners used on the wings of the aircraft and other structural members. The integrity of each joint joined with structural fasteners is of paramount importance to the overall reliability and safety of the aircraft and is directly related to such factors as the geometry (size, shape, diameter) and finish of the hole into which structural fasteners are inserted. These dimensional characteristics must comply with extremely rigid standards due to the effect that the mating between fasteners and fastener holes have on the strength and fatigue life of a joint. Consequently, the precise geometry, finish, and tolerance of both fasteners and fastener holes are critical. This is especially true where the structural member of the aircraft is formed of aluminum alloys which generally are susceptible to nicks from tools or other surface imperfections. Although some imperfections are permissible, dimensional standards for aircraft fastener holes generally require a tolerance of about .+-.0.0015". The resulting inspection and quality control requirements can be both expensive and time-consuming, especially where 100% inspection of the various dimensions of all structurally critical holes is required.
U.S. Pat. No. 5,010,658 to Griffith et al discloses a hole profile gauging system that measures the diameter of a hole at increments along its depth to provide a hole profile. The probe, comprising a split ball-type sensor, is inserted and withdrawn from the hole with measurements taken while the probe is being withdrawn. A transducer senses the separation between the split portions of the probe via a needle inserted between the split portions. By this means, any imperfections or flaws in the wall of the hole are detected by the probe and sent to a processor by the transducer. Measurements obtained are analyzed by the processor which compiles the measurements, generates a profile of the hole, and determines whether the hole is within acceptable tolerance limits. However, the method and apparatus disclosed and chimed in the '658 patent are limited to determining the profile of a cylindrical hole and would not be applicable to tapered holes typically found on aircraft parts.
Other approaches to inspection of fastener holes use a probe with several metal plates which may be energized to create a capacitance between a plate of the probe and the surface of the fastener hole. These approaches take advantage of the well known principle that for two parallel plates, the amount of electrical capacitance is proportional to the area of the plates and inversely proportional to the distance between the plates. This concept is illustrated in FIG. 1 and is embodied in the following equation: Capacitance, C=K(A/d) where plate area A=length 1.times.width w; d=separation distance; and K=proportionality constant. Conductive sensing elements of the capacitive probe serve as one of the parallel plates and the grounded hole wall serves as the other plate.
U.S. Pat. No. 5,021,740 to Sarr et at. discloses a method and apparatus for measuring the distance between a body and a capacitive probe. The apparatus disclosed includes a capacitance-type sensing probe which has a plurality of measuring electrodes and a grounding electrode. The apparatus also includes a control system which comprises a selector, a converter, and a processor. The processor issues control instructions that instruct the selector to select one of the plurality of measuring electrodes. The selected electrode is coupled to the converter and forms a capacitor with a nearby grounded surface of the conductive body and thus provides an associated capacitance value to the converter. The capacitance value is then converted to a time-related signal which in mm is translated into a distance value that is substantially equal to the distance between the capacitance-type sensing probe and the surface of the conductive body. However, the method and apparatus disclosed and claimed in the '740 patent is prone to undesirable influences on the capacitance measurements such as the capacitance between the selected electrode and other electrodes or conductive components. Such residual capacitances may vary with the mere flexing of a cable and contribute significantly to the total measured capacitance. For example, residual capacitances can be on the order of several thousand picofarads making capacitance measurements in the 0.1 picofarad range unreliable. Adding to this problem is the fact that typical capacitance measuring techniques employ digital counters which sample capacitor charging or discharging times at rates of several hundred counts per picofarad requiring more complex, higher capacity, and thus, more expensive electronics to measure the higher capacitances caused by residual capacitances.
As will be appreciated from the above discussion, there exists a need in the aircraft industry for a method and apparatus that is capable of accurately and reliably measuring the distance between a capacitive probe and the surface of a fastener hole especially when very small capacitance values are involved. There is also a need for a method and apparatus for inspecting fastener holes which is fast and easy to use so that the many fastener holes can be inspected and appropriate corrective action immediately taken to fix any holes which do not comply with required dimensional standards or tolerances. It is to the provision of such a method and apparatus that the present invention is primarily directed.